Modern automotive engines contain electronic engine control systems which vary operating parameters of the engine, such as air/fuel ratios and ignition timing, to achieve optimum performance. Such control systems are capable of changing engine operating parameters in response to a variety of external conditions.
A primary function of electronic engine control systems is to maintain the ratio of air and fuel at or near stoichiometry. Electronic engine control systems operate in a variety of modes depending on engine conditions, such as starting, rapid acceleration, sudden deceleration, and idle. One mode of operation is known as closed-loop control. Under closed-loop control, the amount of fuel delivered is determined primarily by the concentration of oxygen in the exhaust gas. The concentration of oxygen in the exhaust gas being indicative of the ratio of air and fuel that has been ignited.
The oxygen in the exhaust gas is sensed by a Heated Exhaust Gas Oxygen (HEGO) sensor. The electronic fuel control system adjusts the amount of fuel being delivered in response to the output of the HEGO sensor. A sensor output indicating a rich air/fuel mixture (an air/fuel mixture above stoichiometry) will result in a decrease in the amount of fuel being delivered. A sensor output indicating a lean air/fuel mixture (an air/fuel mixture below stoichiometry) will result in an increase in the amount of fuel being delivered.
Electronic engine control systems operate in a variety of modes depending on engine conditions, such as starting, rapid acceleration, sudden deceleration, and idle. Under closed-loop control, the amount of fuel delivered is determined primarily by the concentration of oxygen in the exhaust gas. The concentration of oxygen in the exhaust gas being indicative of the ratio of air and fuel that has been ignited. A flexible fuel vehicle is capable of operating on different fuels, such as gasoline, methanol, or a mixture of the two, utilize electronic engine control systems to change the engine operating parameters in response to the type of fuel being delivered to the engine. The engines of these vehicles can be run on any combination of gasoline and up to 85% alcohol. Such systems utilize a Flexible Fuel Sensor (FFS) to detect the type of fuel being delivered to the engine and a computer or controller to calculate the percent of alcohol in the fuel and vary the engine operating parameters accordingly. The computer is programmed to control such emission sensitive factors as spark timing, fuel, exhaust gas recirculation, secondary air injection, idle speed, and canister purge. An example of such a system is disclosed in Curran et al. in U.S. Pat. No. 5,230,322.
The combustion of air/fuel mixtures in internal combustion engines, such as those found in automobiles, produces an exhaust gas stream comprised of various gaseous components. Some of these components, such as hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NO.sub.x), may be termed noxious components. Those skilled in the art will appreciate that oxides of nitrogen refers to both NO and NO.sub.2. Environmental concerns have led to ever stricter regulations concerning the maximum allowed emissions of these particular components. Accordingly, the computer may also be programmed to perform diagnostic routines to verify proper actuator and sensor operation, and perform system checks, and to control a malfunction indicator light (MIL) to inform the driver of any problem and store fault codes for later use by service personnel. There are a number of advantages to be realized where a highly reliable diagnostic system is provided. Not only do lower emissions result from maintaining the systems in proper working order, but greater customer satisfaction arises from being accurately informed as early as possible of a malfunction. The California Air Resources Board (CARB) has adopted regulations for onboard diagnostic systems which require a self-monitoring emission and powertrain control system. When a system or component is found to exceed established emission thresholds or a component is operating outside its manufacturer specified tolerances, a fault code must be stored and a malfunction indicator light on the vehicle instrument cluster must be illuminated. Such an onboard diagnostic system (OBD) is disclosed in U.S. Pat. No. 5,671,141, assigned to the assignee of the present invention.
When a vehicle, with gasoline in the fuel tank, has a fuel containing 85% alcohol added, a blend of less than 85% alcohol will be formed, depending on the amount added and the amount of gasoline in the tank prior to fueling. As the vehicle is driven, the fuel supplied to the engine will transition from the "old" fuel to the "new" blend of fuel. Certain OBD monitors, such as the HEGO and FUEL monitors, are affected by instability in fuel makeup. Therefore, it is inadvisable to perform such monitoring during conditions of changing A/F such as occur during the transition described above.